8,180 research outputs found
Objective Momentum Barriers in Wall Turbulence
We use the recent frame-indifferent theory of diffusive momentum transport to
identify internal barriers in wall-bounded turbulence. Formed by the invariant
manifolds of the Laplacian of the velocity field, the barriers block the
viscous part of the instantaneous momentum flux in the flow. We employ the
level sets of single-trajectory Lagrangian diagnostic tools, the trajectory
rotation average and trajectory stretching exponent, to approximate both
vortical and internal wall-parallel momentum transport barrier (MTB)
interfaces. These interfaces provide frame-indifferent alternatives to classic
velocity-gradient-based vortices and boundaries between uniform momentum zones
(UMZs). Indeed, we find that these elliptic manifold approximations and MTBs
also significantly outperform standard vortices and UMZ interfaces in blocking
diffusive momentum transport.Comment: 26 Pages, 14 Figur
Walls Inhibit Chaotic Mixing
We report on experiments of chaotic mixing in a closed vessel, in which a
highly viscous fluid is stirred by a moving rod. We analyze quantitatively how
the concentration field of a low-diffusivity dye relaxes towards homogeneity,
and we observe a slow algebraic decay of the inhomogeneity, at odds with the
exponential decay predicted by most previous studies. Visual observations
reveal the dominant role of the vessel wall, which strongly influences the
concentration field in the entire domain and causes the anomalous scaling. A
simplified 1D model supports our experimental results. Quantitative analysis of
the concentration pattern leads to scalings for the distributions and the
variance of the concentration field consistent with experimental and numerical
results.Comment: 4 pages, 3 figure
Parabolic resonances and instabilities in near-integrable two degrees of freedom Hamiltonian flows
When an integrable two-degrees-of-freedom Hamiltonian system possessing a
circle of parabolic fixed points is perturbed, a parabolic resonance occurs. It
is proved that its occurrence is generic for one parameter families
(co-dimension one phenomenon) of near-integrable, t.d.o. systems. Numerical
experiments indicate that the motion near a parabolic resonance exhibits new
type of chaotic behavior which includes instabilities in some directions and
long trapping times in others. Moreover, in a degenerate case, near a {\it flat
parabolic resonance}, large scale instabilities appear. A model arising from an
atmospherical study is shown to exhibit flat parabolic resonance. This supplies
a simple mechanism for the transport of particles with {\it small} (i.e.
atmospherically relevant) initial velocities from the vicinity of the equator
to high latitudes. A modification of the model which allows the development of
atmospherical jets unfolds the degeneracy, yet traces of the flat instabilities
are clearly observed
Quasi-Objective Eddy Visualization from Sparse Drifter Data
We employ a recently developed single-trajectory Lagrangian diagnostic tool,
the trajectory rotation average , to visualize
oceanic vortices (or eddies) from sparse drifter data. We apply the to two drifter data sets that cover various
oceanographic scales: the Grand Lagrangian Deployment (GLAD) and the Global
Drifter Program (GDP). Based on the , we develop a
general algorithm that extracts approximate eddy boundaries. We find that the outperforms other available single-trajectory-based
eddy detection methodologies on sparse drifter data and identifies eddies on
scales that are unresolved by satellite-altimetry
Interplay between advective, diffusive and active barriers in (rotating) Rayleigh–Bénard flow
Our understanding of the material organization of complex fluid flows has benefited
recently from mathematical developments in the theory of objective coherent structures.
These methods have provided a wealth of approaches that identify transport barriers
in three-dimensional (3-D) turbulent flows. Specifically, theoretical advances have been
incorporated into numerical algorithms that extract the most influential advective,
diffusive and active barriers to transport from data sets in a frame-indifferent fashion.
To date, however, there has been very limited investigation into these objectively defined
transport barriers in 3-D unsteady flows with complicated spatiotemporal dynamics.
Similarly, no systematic comparison of advective, diffusive and active barriers has
been carried out in a 3-D flow with both thermally driven and mechanically modified
structures. In our study, we utilize simulations of turbulent rotating Rayleigh–Bénard
convection to uncover the interplay between advective transport barriers (Lagrangian
coherent structures), material barriers to diffusive heat transport, and objective Eulerian
barriers to momentum transport. For a range of (inverse) Rossby numbers, we identify each
type of barrier and find intriguing relationships between momentum and heat transport
that can be related to changes in the relative influence of mechanical and thermal forces.
Further connections between bulk behaviours and structure-specific behaviours are also
developed
Slow decay of concentration variance due to no-slip walls in chaotic mixing
Chaotic mixing in a closed vessel is studied experimentally and numerically
in different 2-D flow configurations. For a purely hyperbolic phase space, it
is well-known that concentration fluctuations converge to an eigenmode of the
advection-diffusion operator and decay exponentially with time. We illustrate
how the unstable manifold of hyperbolic periodic points dominates the resulting
persistent pattern. We show for different physical viscous flows that, in the
case of a fully chaotic Poincare section, parabolic periodic points at the
walls lead to slower (algebraic) decay. A persistent pattern, the backbone of
which is the unstable manifold of parabolic points, can be observed. However,
slow stretching at the wall forbids the rapid propagation of stretched
filaments throughout the whole domain, and hence delays the formation of an
eigenmode until it is no longer experimentally observable. Inspired by the
baker's map, we introduce a 1-D model with a parabolic point that gives a good
account of the slow decay observed in experiments. We derive a universal decay
law for such systems parametrized by the rate at which a particle approaches
the no-slip wall.Comment: 17 pages, 12 figure
Tick-borne Thogoto virus infection in mice is inhibited by the orthomyxovirus resistance gene product Mx1
We show that tick-transmitted Thogoto virus is sensitive to interferon- induced nuclear Mx1 protein, which is known for its specific antiviral action against orthomyxoviruses. Influenza virus-susceptible BALB/c mice (lacking a functional Mx1 gene) developed severe disease symptoms and died within days after intracerebral or intraperitoneal infection with a lethal challenge dose of Thogoto virus. In contrast, Mx1-positive congenic, influenza virus- resistant BALB·A2G-Mx1 mice remained healthy and survived. Likewise, A2G, congenic B6·A2G-Mx1 and CBA·T9-Mx1 mice (derived from influenza virus- resistant wild mice) as well as Mx1-transgenic 979 mice proved to be resistant. Peritoneal macrophages and interferon-treated embryo cells from resistant mice exhibited the same resistance phenotype in vitro. Moreover, stable lines of transfected mouse 3T3 cells that constitutively express Mx1 protein showed increased resistance to Thogoto virus infection. We conclude that an Mx1-sensitive step has been conserved during evolution of orthomyxoviruses and suggest that the Mx1 gene in rodents may serve to combat infections by influenza virus-like arboviruses.</p
Opportunities and Challenges in Export Horticulture as an Agro-industrial Food System: Case Study of Northwest Mount Kenya Region
Export horticulture in Kenya viewed as an agro-industrial food system is currently the fastest growing agricultural sub-sector in terms of foreign exchange earnings. Increased demand for horticulture products led to production spreading beyond the traditional mountainous high yielding areas into arid and semi-arid zones as in Northwest Mount Kenya. This food system competes with other food systems for common pool resources needed for the production of food. We argue that local actors, especially poorer households lack the power to influence the institutions (‘rules of the game’) of production and resource ownership by which the dominant agro-industrial system impacts their livelihoods.This paper is structured to include the following sections: the introduction, materials and methods, results, discussion and conclusions on the challenges and opportunities in export horticulture as an agro-industrial food system: case study of Northwest Mount Kenya region
MxA Gene Expression after Live Virus Vaccination: A Sensitive Marker for Endogenous Type I Interferon
MxA gene expression is known to be regulated tightly and exclusively by type I interferons (IFNs). The kinetics of MxA gene expression was analyzed in peripheral blood mononuclear cells from 11 healthy volunteers vaccinated with the 17-D strain of yellow fever virus. A reliable induction of MxA RNA and MxA protein was found in the absence of easily detectable serum IFN activity. Thus, steady-state MxA RNA levels were elevated 8- to 30-fold above prevaccination levels on day 5 after vaccination. The average increase of MxA protein was ∼50-fold. In contrast, no induction of MxA RNA or MxA protein was detectable in 3 similarly vaccinated controls who were immune because of previous vaccinations. The IFN marker 2′-5′-oligoadenylate (2-5A) synthetase known to react to both type I and type II IFNs showed a similar response but did not differentiate equally well between nonimmune and immune vaccinees. β2-microglobulin and neopterin reacted poorly, remaining at low levels within the normal range. These results demonstrate that MxA gene expression is a good marker for detecting minute quantities of biologically active type I IFN during viral infection
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